Here is the full list of all 12 Song of the Myrne: What Lies Beneath achievements.
Read a certain grimoire
Defeat the final boss
Defeat The Giant Plant
Beat the Spider's Forest boss. Spoiler: it's a spider
Defeat The Lich
Defeat The Troll
Craft an item
Walk Henry with a leash
Unlock all magical skill trees
Have an item equiped in each of your appearance equipment slots at the same time
Finish the story "The Sorcerer"
Find the secret ending of "The Sorcerer"